The application of coating materials using electrostatic spraying techniques has been practiced in industry for many years. In these applications, the coating material is discharged in atomized form and an electrostatic charge is imparted to the atomized particles which are then directed toward a substrate maintained at a different potential to establish an electrostatic attraction for the charged atomized particles. As described in detail in U.S. Pat. Nos. 5,078,168 and 5,221,194, both owned by the assignee of this invention, the recent trend has been to shift away from the use of solvent-based coating materials, e.g. varnishes, lacquers, enamels, and the like, in favor of water-based coatings which reduce problems of explosiveness and toxicity which had plagued solvent-based coatings for many years. Unfortunately, this switch from electrostatically spraying solvent-based coatings to those of the water-based type has sharply increased the risk of electrical shock among system operators.
The problem of electrical shock from water-based coatings is addressed in U.S. Pat. Nos. 5,078,168 and 5,221,194 wherein a "voltage-block" system is provided for transferring electrically conductive coating materials without the formation of a completed electrical path between the source of coating material and the high voltage electrostatic power supply. The system in U.S. Pat. No. 5,078,168 comprises first and second shuttle devices which are serially connected to two large reservoir piston pumps. The first shuttle is movable between a transfer position and a neutral position relative to a filling station which is connected to a source of electrically conductive coating material. At the filling station, the first shuttle is operative to transfer coating material from the source into the reservoir of the first pump. In the neutral position, the first shuttle is electrically isolated, i.e. physically spaced, from the filling station. The second shuttle device is movable between a transfer position wherein it interconnects the first piston pump with the second piston pump, and a neutral position wherein the two pumps are electrically isolated from one another and the second piston pump supplies Coating material to the dispensers. Movement of the shuttles is controlled to maintain one of the shuttles in a neutral position at all times during a coating operation so that there is never a completed electrical path between the source of electrically conductive coating material and the electrostatically charged coating material at the dispenser(s).
One problem with apparatus of the type disclosed in U.S. Pat. No. 5,078,168 involves the pressure available to discharge the coating material from the reservoir of the second piston pump. Each of the first and second reservoir pumps includes a piston which is movable in one direction in response to the application of air pressure thereagainst to discharge coating material from the reservoir, and is movable in the opposite direction as new coating material is added to the reservoir. In order to permit filling of the reservoir of the second pump with coating material supplied from the first pump, the air pressure applied to the piston in the second pump must be reduced compared to that of the first pump otherwise the piston within the second pump would not move and allow the reservoir therein to be filled. Because of this reduced pressure level within the second pump, the coating material is discharged therefrom at a relatively low pressure level. As a result, comparatively few coating dispensers can be supplied with coating material from the second pump, and the spray pattern emitted from such dispensers is not always stable.
Another problem with voltage-block systems of the type disclosed in U.S. Pat. No. 5,078,168, is a relatively wide pressure fluctuation in the coating material discharged from the second pump to the coating dispenser(s). When the reservoir of the second pump is filled and coating material is discharged by its piston moving in a downward direction toward the base of the reservoir, the fluid pressure output from the second pump is less than the air pressure at which the piston is forced downwardly because the seal friction with which the piston seals against the side walls of the pump reservoir opposes downward motion of the piston. This produces a comparatively low fluid discharge pressure, significantly lower than the air pressure, with the attendant disadvantages noted above. On the other hand, a higher fluid discharge pressure, e.g. higher than the air pressure, is output from the second pump when it is filled with coating material from the first pump. This is because the fluid pressure of the coating material introduced at the base of the second pump, on the bottom side of the piston, must overcome both the air pressure acting on the opposite or top side of the piston and the seal friction of the piston seals against the side wall of the piston reservoir. Since the air pressure in the system remains constant, the fluid pressure fluctuates depending on whether the piston within the second pump is moving upwardly or downwardly. Accordingly, a potentially large pressure fluctuation can occur at the discharge side of the second pump depending upon whether or not the second pump is undergoing a fill cycle or a discharge cycle when coating material is discharged therefrom to the coating dispenser(s). Such pressure fluctuation limits the number of dispensers which can be supplied by the second pump, and/or adversely affects the spray pattern obtained from such dispensers.
These problems of adequate pressure at the coating dispensers and pressure fluctuation from the second piston pump have been addressed in U.S. Pat. No. 5,326,031 to Konieczynski, entitled "Apparatus for Dispensing Conductive Coating Materials Including Color Changing Capability," which is owned by the assignee of this invention. In this system, electrically conductive coating material is transmitted from two "parallel" flow paths to one or more coating dispensers. Each flow path comprises a voltage-block construction including a transfer unit having a filling station connected to the source(s) of coating material, a discharge station spaced from the filing station and a shuttle movable between and releasably coupled to the filing station and to the discharge station. Upon movement of the shuttle to the filling station of the transfer unit within one of the two flow paths, the shuttle is effective to transfer coating material from the source into the reservoir of a piston pump associated with such flow path. When the reservoir of the piston pump is filled, the shuttle moves and is coupled to the discharge station wherein a connection is made allowing the coating material to be transferred from the pump reservoir, through the discharge station of the transfer unit, and, into a "sync" valve connected to the dispensers. This sync valve is common to both flow paths and is effective to switch the flow of coating material to the dispensers from one flow path to the other. The operation of the system is synchronized such that when the pump of one flow path is supplying coating material to the dispensers, the pump of the other flow path is receiving coating material from the source. A voltage block is continuously maintained between the source and charged dispensers, and the dispensers can be essentially continuously supplied with coating material from one or the other of the parallel flow paths.
The system of U.S. Pat. No. 5,326,031 described above has proven to be highly successful for large, multiple gun systems intended for high volume, large quantity coating operations. But in low volume, single gun systems used in comparatively small manufacturing facilities, the parallel flow system of U.S. Pat. No. 5,326,031 provides more capacity than is required and is too costly.